Rigid, thin-film disks are the primary storage medium for digital data used in computer disk drives. A rigid, thin-film disk recording medium includes a rigid substrate that is coated with a storage material, such as a magnetic or optical material. The storage capacity of these recording media can range from a few hundred kilobytes to several gigabytes. As technology advances, so does the need for rigid, thin-film disk recording media with increased storage capacity. One way of increasing the storage capacity of a rigid, thin-film disk is to maximize the surface area of the disk by extending recordable tracks to the outer edge of the disk. Tracks near the outer edge of a disk have a larger circumference and are, therefore, capable of storing more data than those located closer to the center of the disk.
In the manufacture of rigid, thin-film disk recording media, prior to their coating with a storage material, a conventional grinding apparatus grinds the surfaces of the rigid substrates to a desired finish. One such conventional grinding apparatus is a planetary grinder having two rotating platens: an upper platen; and a lower platen. Each platen includes a grinding wheel made up of several grinding stones for grinding a surface of the rigid substrates. A sun gear and a ring gear are mounted in close proximity to the lower platen. The grinder further includes a plurality of substrate carriers disposed between the grinding wheels of the platens. The substrate carriers, each of which is capable of carrying several substrates, are coupled to the sun and ring gears of the grinder. In operation, the substrate carriers are loaded with rigid substrates, and the upper platen is lowered onto the substrate carriers, such that the grinding stones of the upper and lower platens contact the respective upper and lower surfaces of the substrates. As the sun gear and/or ring gear rotates, the substrate carriers simultaneously revolve around the sun gear and rotate about their own axis between the two grinding wheels, thereby grinding the surfaces of the substrates.
Over time, the grinding stones become worn and require dressing to restore them to their original quality. Grinding stones are typically dressed using a conventional ring dresser. The ring dresser is an annular disk, similar in size to a substrate carrier, with teeth along its outer circumference and diamond pellets on both planar surfaces. During the dressing process, at least three substrate carriers are replaced with the ring dressers, and the upper platen is lowered until both grinding wheels are in contact with the ring dressers. As the sun gear is rotated, the ring dressers revolve around the sun gear and rotate about their axes to dress the grinding stones.
One problem with conventional ring dressers is that they fail to dress the grinding stones properly, resulting in rigid substrates with a high outer diameter (OD) radial curvature. FIG. 1 illustrates the profile of a lower platen grinding stone immediately after being dressed with a conventional ring dresser. Portions of the grinding stone along the edges proximate the sun and ring gears receive less exposure to the ring dressers than areas in between. Because the ring dressers have the same dimensions as the substrate carriers, only the outer portion of the ring dressers dress the innermost and outermost portions of the grinding stone, and less material is removed from these areas of the grinding stone. As a result, the inner and outer edges of the grinding stone are highly aggressive and will remove more material from the rigid substrates than other portions of the grinding stone. These grinding stones will produce rigid substrates with a high OD radial curvature, because only the outermost edge of the substrates contacts the inner and outer edges of the grinding stones as the substrate carriers rotate and revolve around the sun gear. A high OD curvature, also referred to as "radial curvature," "roll off" or "dub off," significantly limits the storage capacity of a rigid, thin-film disk recording medium, because it reduces the number of recordable tracks along the outer edge of the disk.
Two known methods of addressing the OD curvature problem associated with ring dressers include the use of low grinding pressures and the use of toleranced carriers. The first method, operating at low grinding pressures, is an inefficient use of the grinder. Low grinding pressures extend run times, thereby decreasing the machine's throughput. In addition, low grinding pressures can result in premature grinding stone loading and increased costs.
The second method employs toleranced carriers. Toleranced carriers are substrate carriers that have a thickness very close to the desired thickness of the rigid substrate. The grinding apparatus grinds the substrates down to the toleranced carriers. Toleranced carriers, however are difficult to manufacture. In addition, because the grinding stones apply pressure loads to the toleranced carriers rather than to the rigid substrates, toleranced carriers extend run times and decrease throughput. Moreover, toleranced carriers are subject to wear and must be routinely replaced.
Thus, there is a need for a dresser which provides for preferential shaping of the grinding stones. In particular, the dresser should eliminate regions of high aggressiveness at the inner and outer edges of the grinding stones.